1. Field of the Invention
The present invention generally relates to a liquid crystal display device, and particularly, to a liquid crystal display device operating in a vertical (homeotropic) alignment mode.
2. Description of the Related Art
Because a liquid crystal display device has low power consumption and can be made compact, it is widely used in various portable information processing devices, such as a laptop computer, or a cellular phone. On the other hand, so far, performance of the liquid crystal display device has been improved significantly, and the latest liquid crystal display devices have such high response speed and contrast ratio that they can be used in a desktop computer or a workstation to replace conventional CRT (Cathode Ray Tube) display.
In the related art, usually, a TN (Twisted Nematic) type liquid crystal operating in a normally-white mode is used in practical liquid crystal display devices. In such a TN mode liquid crystal display device, the direction of alignment of the liquid crystal molecules in the plane of a liquid crystal layer changes in response to a driving voltage signal applied on the liquid crystal layer, and by controlling changes of the direction of alignment of the liquid crystal molecules, the transmission light is switched on or switched off.
This TN mode liquid crystal display device, however, is limited in a contrast ratio. This limitation can be attributed to the operation principle of the TN mode liquid crystal display device. In addition, it is difficult for the TN mode liquid crystal display device to provide a wide viewing angle, which is required by, for example, a desktop display.
Inventors of the present inventions have proposed a so-called vertical-alignment liquid crystal display device, that is, a liquid crystal display device in which the liquid crystal molecules are aligned along a direction substantially perpendicular to the liquid crystal layer when a driving voltage signal is not applied (that is, an un-driven state).
FIG. 1A and FIG. 1B are schematic perspective views illustrating operation principle of the vertical-alignment liquid crystal display device 10 (also referred to as a MVA (multi-domain vertical alignment) liquid crystal display device), proposed by the inventors of the present invention. Specifically, FIG. 1A shows the liquid crystal display device 10 in the un-driven state, that is, the driving voltage is not applied to the liquid crystal display device 10, and FIG. 1B shows the liquid crystal display device 10 in a driven state, that is, the driving voltage is applied to the liquid crystal display device 10.
As illustrated in FIG. 1A, a liquid crystal layer 12 is interposed between a glass substrate 11A and a glass substrate 11B. The glass substrates 11A, 11B, and the liquid crystal layer 12 constitute a liquid crystal panel.
Although not illustrated, molecule alignment films are arranged on the glass substrate 11A and glass substrate 11B, respectively. Because of the molecule alignment films, the liquid crystal molecules in the liquid crystal layer 12 are aligned along a direction substantially perpendicular to the liquid crystal layer 12 when the driving voltage signal is not applied (that is, the un-driven state). In this state, a polarization plane of a light beam incident to the liquid crystal device essentially does not rotate in the liquid crystal layer 12. Therefore, in the un-driven state shown in FIG. 1A, if a polarizer and an analyzer are arranged above and below the liquid crystal panel in a crossed-Nicol configuration, the light beam passing through the polarizer and incident on the liquid crystal layer 12 is blocked by the analyzer.
On the other hand, in the driven state shown in FIG. 1B, the liquid crystal molecules are tilted due to the applied electrical field, and the polarization plane of the light beam incident to the liquid crystal layer 12 rotates in the liquid crystal layer 12. Thus, the light beam passing through the polarizer and incident on the liquid crystal layer 12 is allowed to pass through the analyzer.
In the liquid crystal display device 10, during a transition from the un-driven state to the driven state, in order to regulate the tilting direction of the liquid crystal molecules so as to improve the response speed of the liquid crystal panel, projecting patterns 13A, 13B are arranged in parallel to each other on the glass substrate 11A and the glass substrate 11B. By providing the projecting patterns 13A, 13B, the response speed of the liquid crystal device 10 is increased, at the same time, different domains involve different tilting directions of the liquid crystal molecules in the liquid crystal layer, as a result, the viewing angle of the liquid crystal device 10 is widened.
FIG. 2A and FIG. 2B are schematic views illustrating operation principle of a vertical alignment liquid crystal display device 20 of the related art, which is proposed by the inventors of the present invention.
In Japanese Laid Open Patent Application No. 2002-107730, the inventors of the present invention proposed a vertical alignment liquid crystal display device 20, as illustrated in FIG. 2A and FIG. 2B, in which stripe patterns 24 are arranged in parallel to each other, and the stripe patterns 24 form a periodically varying electrical field in the liquid crystal layer 22, and due to the electrical field, liquid crystal molecules 22A are pre-tilted along the direction in which the stripe patterns 24 extend.
In addition, in Japanese Laid Open Patent Application No. 2002-107730, the inventors of the present invention also proposed a vertical alignment liquid crystal display device 40, which corresponds to a combination of the above vertical alignment liquid crystal display device 10 and the vertical alignment liquid crystal display device 20.
As illustrated in FIG. 2A, basically, the liquid crystal display device 20 includes a glass substrate 21A and a glass substrate 21B with a liquid crystal layer 22 being interposed in between. Electrode layers 23A and 23B are provided on the glass substrates 21A and 21B, respectively.
In addition, fine structure patterns 24 are provided on the surface of the electrode layer 23A to modify the pattern of the electrical field generated between the electrode layers 23A and 23B. On the glass substrate 21A, a molecule alignment film 25MA is formed on the surface of the electrode layer 23A to cover the fine structure patterns 24. On the glass substrate 21B, a molecule alignment film 25MB is formed to cover the electrode layer 23B.
The molecule alignment films 25MA, 25MB are in contact with the liquid crystal layer 22, and the liquid crystal molecules 22A in the liquid crystal layer 22 are aligned along a direction substantially perpendicular to the liquid crystal layer 22 when the electrical field is not applied between the electrode layer 23A and the electrode layer 23B (that is, the un-driven state).
A polarization film 26A, which has a first optical absorption axis and acts as a polarizer, is provided on a lower main surface of the glass substrate 21A, while a polarization film 26B, which has a second optical absorption axis perpendicular to the first optical absorption axis and acts as another polarizer, is provided on an upper main surface of the glass substrate 21B.
In the example illustrated in FIG. 2A, the fine structure patterns 24 are conductive or insulating fine projecting patterns arranged in parallel to each other on the electrode layer 23A, but the fine structure patterns 24 may also have other configurations as long as it is able to locally modify the electrical field in the liquid crystal layer 22.
FIG. 3 is a schematic view illustrating operation principle of another example of the vertical alignment liquid crystal display device 20 of the related art.
As illustrated in FIG. 3, the fine structure patterns 24 may also be fine depressed patterns such as plural cutouts in parallel to each other in the electrode layer 23A. In FIG. 3, the same reference numbers are used for the same elements as in FIG. 2A and FIG. 2B, and overlapping descriptions are omitted.
If the fine structure patterns 24 include projecting patterns on the electrode layer 23A, as illustrated in FIG. 2A, preferably, the fine structure patterns 24 are formed from a transparent material so that a light beam incident into the liquid crystal display device can pass through the fine structure patterns 24.
Return to FIG. 2B, FIG. 2B shows the driven state of the liquid crystal display device 20, that is, the driving voltage is applied between the electrode layers 23A and 23B to change the direction of alignment of the liquid crystal molecules 22A on the glass substrate 21A.
As illustrated in FIG. 2B, in the driven state of the liquid crystal display device 20, because of the effect of the electrical field locally modified by the fine structure patterns 24, the liquid crystal molecules 22A are aligned to be tilted toward the extending directions of the fine structure patterns 24.
In the liquid crystal display device 20, when the driving voltage is applied between the electrode layers 23A and 23B, and the driving electrical field is formed in the liquid crystal molecule layer 22, because each liquid crystal molecule 22A is tilted toward the extending directions of the fine structure patterns 24 in response to the electrical field modified by the fine structure patterns 24, the response speed of the liquid crystal display device 20 is greatly improved compared with the liquid crystal display device 10 shown in FIG. 1A and FIG. 1B, because in the liquid crystal display device 10 shown in FIG. 1A and FIG. 1B, the tilt of the liquid crystal molecules has to propagate from regions near the projecting patterns 13A, 13B to other regions, but in the liquid crystal display device 20 in FIG. 2B, this is not necessary.
In addition to the above advantages, from FIG. 2B, it is found that in the liquid crystal display device 20, the alignment directions of the liquid crystal molecules 22A are essentially restricted to the extending directions of the fine structure patterns 24 in the driving state, therefore, the twisted angle of each liquid crystal molecule 22A does not change even when interactions between the tilted liquid crystal molecules 22A are present, and this results in display of high contrast ratio and high quality.
When the driving voltage is applied between the electrode layers 23A and 23B, the fine structure patterns 24 form electrical field in the liquid crystal layer 22, which is uniform in a first direction along the extending directions of the fine structure patterns 24, and varies periodically in a second direction perpendicular to the first direction.
FIG. 4 is a plan view of the substrate 21A in FIG. 3, illustrating an example of a configuration of a liquid crystal display device of the related art. In FIG. 4, the same reference numbers are used for the same elements as those shown in FIG. 3.
As illustrated in FIG. 4, on the substrate 21A, a thin film transistor (TFT) 21T is formed at the cross point between a scanning electrode 22S and a data electrode 22D which are formed below the pixel electrode 23A, and the pixel electrode 23A is connected with the TFT 21T. The substrate 21A is also referred to as a TFT substrate.
On the pixel electrode 23A, the fine structure patterns 24 are patterned to be in parallel at intervals 24G. On the pixel electrode 23A, large gaps 25A, which corresponds to the structure 13A in FIG. 1A and FIG. 1B, are patterned in a zigzag manner. Due to this, a pixel region in FIG. 4 is divided into an upper domain region and a lower domain region, and the tilting directions of the crystal liquid molecules in the upper domain region and the lower domain region are perpendicular to each other.
In FIG. 4, resist film structures 25B are formed in a zigzag manner on the substrate 21B facing the substrate 21A. The structures 25B are in correspondence to the projecting structure 13B in FIG. 1A and FIG. 1B.
In the configuration shown in FIG. 4, because of the fine structure patterns 24 and the gaps 24G between the stripe patterns 24, the tilting directions of the liquid crystal molecules are essentially regulated to be along the extending directions of the gaps 24G. Further, the pre-tilt angles are defined by the structures 25A and 25B. Hence, this configuration exhibits a high response speed.
In the configuration shown in FIG. 4, between the upper domain region and the lower domain region, auxiliary capacitance Cs is produced by the electrode pattern 23C.
Listed below are references which disclose techniques related to the present invention:
Japanese Laid-Open Patent Application No. 2002-107730,
Japanese Laid-Open Patent Application No. 2002-287158,
Japanese Laid-Open Patent Application No. 2000-305086, and
Japanese Patent Gazette No. 3456896.
FIG. 5 is a plan view of another example of the TFT substrate 21A of a liquid crystal display device of the related art, having the configuration shown in FIG. 4. In FIG. 5, the same reference numbers are used for the same elements as those shown in FIG. 3 and FIG. 4.
As illustrated in FIG. 5, plural configurations as shown in FIG. 4 are arranged in correspondence to red (R), green (G), and blue (B) colors, respectively, and on the TFT substrate 21B facing the TFT substrate 21A, a color filter is arranged in correspondence to the configuration on the TFT substrate 21A as shown in FIG. 5.
The structures 25B, one of which is indicated by “BB” in FIG. 5, are arranged to pass through corners of the pixel electrodes 23A on which the fine structure patterns 24 are formed. The structures 25A, that is, the cutouts formed in the pixel electrodes 23A, as indicated by “AA” in FIG. 5, bend at edges of the pixel electrodes 23A.
FIG. 6A and FIG. 6B are diagrams illustrating relation between the structures 25A, 25B and alignment of the liquid crystal molecules 22A in the liquid crystal display device shown in FIG. 5, especially, the liquid crystal molecules 22A at an edge of the pixel electrode 23A.
The structure 25A and 25B have the function of inducing pre-tilt of the liquid crystal molecules 22A, as described above. However, at the edge of the pixel electrode 23A, because of interaction between the effect of the edge of the pixel electrode 23A and the effect of the structure 25A and 25B, alignment of the liquid crystal molecules 22A is disordered.
Nevertheless, when the bending point of the structure 25A is at the edge of the pixel electrode 23A, as shown in FIG. 6A, or when the structure 25B passes through the corner of the pixel electrode 23A, as shown in FIG. 6B, the effect of the structure 25A or 25B of constraining alignment of the liquid crystal molecules 22A is substantially in agreement with the effect of the edge of the pixel electrode 23A of constraining alignment of the liquid crystal molecules 22A, and in such regions, that is, near the edge or corner of the pixel electrode 23A, alignment of the liquid crystal molecules 22A is not disordered.
FIG. 7A and FIG. 7B, continuing from FIGS. 6A and 6B, are diagrams illustrating relation between the structure 25A, 25B and alignment of the liquid crystal molecules 22A in the liquid crystal display device shown in FIG. 5, especially, the liquid crystal molecules 22A at an edge of the pixel electrode 23A.
When the bending point of the structure 25A is not at the edge of the pixel electrode 23A, as shown in FIG. 7A, or when the structure 25B does not pass through the corner of the pixel electrode 23A, as shown in FIG. 7B, the effect of the structure 25A or 25B of constraining alignment of the liquid crystal molecules 22A is not in agreement with the effect of the edge of the pixel electrode 23A of constraining alignment of the liquid crystal molecules 22A, and as a result, alignment of the liquid crystal molecules 22A is greatly disordered, and this results in a black spot as shown in FIG. 7A and FIG. 7B.
In this way, it is ideal if the arrangement shown in FIG. 6A and FIG. 6B is formed. In practice, however, pitches of the structures 25A and 25B are defined according to the response speed and transmission of the liquid crystal display device, and it is difficult to realize the ideal arrangement of the structures 25A and 25B in a practical liquid crystal display device.
In a liquid crystal display device of the related art, spacers, such as silica beads having specified diameters are used in order to maintain the thickness of the liquid crystal layer, that is, the thickness of the liquid crystal cell, to be a preset value.
On the other hand, recently, a structure of a liquid crystal panel is proposed which does not involve a step of distributing such spacers can be omitted when being fabricated, and this eliminates the problem of non-uniform display caused by non-uniform distribution density of the spacers.
For example, Japanese Laid Open Patent Application No. 11-242211 discloses a columnar spacer extending between a first substrate and a second substrate. Such a columnar spacer may be formed by first depositing a resist film or a polyimide film on either the first substrate or the second substrate, and then patterning the resist film or the polyimide film to a predetermined thickness, thereby, obtaining the columnar spacer of a desired shape at a desired position.
Similar to the projecting patterns 13A, 13B in FIG. 1A and FIG. 1B, such a spacer formed from an organic film has the function of constraining the alignment of the liquid crystal molecules in the liquid crystal layer. Because of the alignment constraining function of such a columnar spacer, a multi-domain structure is proposed in, for example, Japanese Laid Open Patent Application No. 2002-287158, in which the above columnar spacer is arranged at the center of a pixel electrode, and each pixel is divided into multiple sector-shaped domains with the columnar spacer as a center.
On the other hand, because the liquid crystal layer cannot be distributed in a region where the columnar spacer is arranged, such a region cannot be used for display. Hence, it is preferable that the columnar spacer be arranged not in the pixel region. For example, in the above mentioned Japanese Laid Open Patent Application No. 11-242211, plural columnar spacers are arranged symmetrically around a pixel region, and a symmetric domain structure as desired is formed in the pixel region.
When the columnar spacer is arranged in a liquid crystal display device having TFT substrates as illustrated in FIG. 4 or FIG. 5, because the columnar spacer has the function of constraining the alignment of the liquid crystal molecules, the same problems as explained with reference to FIG. 7A and FIG. 7B may occur.
FIG. 8 is a plan view illustrating an example of a configuration of a liquid crystal display device 30A of the related art, which is obtained by providing a columnar spacer P in the liquid crystal display device 30 having the TFT substrate 21A. In FIG. 8, the same reference numbers are used for the same elements as those shown in FIG. 4.
As illustrated in FIG. 8, the columnar spacer P is formed on an electrode pattern 23C across a center portion of the pixel electrode 21A corresponding to the structure 25A, in other words, the columnar spacer P is formed out of the visible region of the liquid crystal display device 30A. However, as illustrated in FIG. 8, while it is desired that the direction of alignment of the liquid crystal molecules 22A be parallel to the extending direction of the fine structure 24, because of presence of the columnar spacer P, in practice, the alignment direction of the liquid crystal molecules 22A near the spacer P turns out to be roughly perpendicular to the extending direction of the stripe patterns of the fine structure 24.
In this situation, even though the columnar spacer P is formed out of the visible region of the liquid crystal display device 30A, it causes a dark portion on a display region and can be recognized by viewers. Furthermore, in the structure shown in FIG. 8, the columnar spacer P causes an additional disorder of the alignment of the liquid crystal molecules 22A besides the disorder of the alignment of the liquid crystal molecules 22A as explained in FIG. 7A and FIG. 7B.